The copilot of a Sichuan Airlines flight from Chongqing, China, to Lhasa, Tibet, was nearly sucked out of the plane Monday morning after the plane’s right-hand windshield suddenly burst and fell off, Chinese authorities said.

The pilot said he looked across the cockpit and saw his co-pilot ‘‘partially hanging out of the window,” saved only by his seat belt, the newspaper Chengdu Economic Daily reported.

Even though it’s happened twice in two months, incidents of people being sucked out of plane windows are extremely rare, experts said. Federal Aviation Administration spokesman Lynn Lunsford said such incidents are a subset of aviation accidents that, as a whole, have become very unusual.

Get Fast Forward in your inbox:

Forget yesterday's news. Get what you need today in this early-morning email.

“Up until the Southwest Airlines accident, we had gone from 2009 until that day, basically nine years, without a single fatality in US commercial aviation,” he said.

But what happens in such incidents and why?

When a window-sized hole opens in an airplane, there is a strong force that sucks air out, experts say. That’s because airplanes are pressurized so people can survive in the thin atmosphere at the tremendous heights where planes travel. The key is the difference between the pressure inside the cabin and outside.

When a plane is cruising at 40,000 feet, for example, the outside air pressure can be 2.73 pounds per square inch while the inside pressure is at 11.23 pounds per square inch, said George Bibel, a mechanical engineering professor at the University of North Dakota and author of “Plane Crash: The Forensics of Aviation Disasters.”

Advertisement

That’s a difference of 8.5 pounds per square inch (around the maximum for most planes). If a hole with an area of 100 square inches opens, then there will be 850 pounds of force pushing, or being sucked, out of the hole.

The force is most intense the more you are blocking the hole, Bibel said.

“The closer you are the more dangerous it is,” he said. “The worst-case scenario is blocking the hole like a plug.” That would bring the entire 850 pounds of force in the example to bear.

R. John Hansman, an MIT aeronautics professor, said, “The danger zone is if you get sucked up into the hole.”

But that force will diminish, Hansman said, even if you are just a little distance away from the hole. He likened the effect to putting your hand over the end of a vacuum cleaner. If you put your hand right over the vacuum cleaner pipe, blocking it, the force will be strong. Just an inch away the force is much, much smaller.

Advertisement

At that point, “the thing that’s pushing you is the wind force that’s going into the hole,” he said.

He estimated that it would be dangerous to be within 10 to 15 inches of a window-sized hole.

In addition to distance, another factor at work in passengers’ favor is the reduction in cabin pressure, the experts said. A hole will release pressure and as pressure declines, getting closer to the outside pressure, the force sucking someone out of a plane will decline.

“Fairly quickly, the pressure will start to drop in the cabin and the amount of force will go down,” Hansman said.

Hansman said a window-sized hole is the most dangerous scenario on a plane for people being sucked out of the plane.

As for a larger-sized hole, like the mammoth one that opened up 30 years ago on an Aloha Airlines plane, killing one flight attendant, the pressure would rapidly equalize inside and outside so being sucked out would not be an issue. (He noted that people in that case would be subject to a different kind of danger, however: a 300-mile-per-hour wind blowing through the cabin.)

While it’s extremely rare for people to get sucked out of airplanes, they can easily take a simple precaution that will protect them both from that and from much more common turbulence, experts said.

“The FAA recommends people wear their seat belts any time they’re in their seat, start to finish. Clear air turbulence is one of the leading causes of injury in aviation,” said Lunsford.

“Just keep your seat belt on and it’s not a problem,” Bibel said. “Flying is significantly safer than driving. It always has been. It always will be.”